Why do layered materials such as mica compress differently in different directions?

Layered materials like mica exhibit anisotropy—significant differences in compression depending on the direction of applied force—due to the variation in bond strength within their crystal structure.

Mica is composed of alternating layers:
* Strong layers: These contain rigid, relatively incompressible bonds, such as silicon-oxygen, aluminum-oxygen, and magnesium-oxygen bonds.
* Weak layers: These consist of alkali cations that join the stronger double layers together.

Because bond compression is strongly dependent on bond strength, the material is much more compressible perpendicular to these layers than within them. In fact, the compressibility of mica is five times greater in the direction perpendicular to the layers than it is within the layers themselves. This differential response to pressure is a fundamental characteristic of layered silicates and contributes to the development of distinct textures in metamorphic rocks like schist.2